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The following quote was obtained by the news editors from the background information supplied by the inventors: "Nanofibrillar cellulose (NFC) consists of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. NFC is based on a natural polymer that is abundant in nature. Nanofibrillar cellulose has many potential uses for example based on its capability of forming viscous gel in water (hydrogel).
"NFC production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. The concentration of NFC in dispersions is typically very low, usually around 1-5%. After the grinding process, the obtained NFC material is a dilute viscoelastic hydrogel. The material itself is usable as such in many applications, but logistic costs are too high to transport the material from the production site. In some applications, the high water content is not acceptable, i.e. the formulations do not tolerate large amounts of water.
"Thus, there is an evident need for increasing the concentration of the final product so that the transport costs would be decreased and the NFC could be used in the final destination at a suitable concentration desired by the end user by simply redispersing the NFC in water.
"Concentration or drying of NFC hydrogel is challenging, however. The specific surface area of NFC is very high due to its nanoscopic dimensions. Respectively, strong water retention is natural for NFC since water is bound on the surfaces of the fibers through numerous hydrogen bonds. Conventional separation techniques, such as filtration or evaporation are not feasible with NFC hydrogels, at least not on industrial level. The problem is widely recognized and heavily studied but not really solved.
"The fundamental problem in mechanical water removal is the ability of NFC hydrogel to form a very dense and impermeable nanoscale membrane around itself, for example during filtration. The formed shell prevents diffusion of water from the gel structure, which leads to very slow concentration rates. The same applies to vacuum evaporation where the skin formation blocks the evaporation of water.
"Another problem in drying of NFC is the non-redispersibility of the dried nanofibers. During the water removal, the NFC-water bonds are replaced with NFC-NFC interactions and the fibers are permanently aggregated. This can be prevented with the use of certain additives during the drying stage, such as CMC, or by chemical modification of the microfibril surface, e.g. oxidation or carboxymethylation. With those methods NFC can be re-activated after complete drying.
"In the literature, the use of organic solvents in separation of MFC from water has been described. The proposed processes have been based on precipitation of dilute NFC dispersion into a non-solvent, such as isopropanol. Precipitation is typically carried out from dilute solutions with high speed mixing.
"For example international publication WO0166600 describes a process where quaternary amine functionalized cellulose gel is added to isopropanol while stirring, at a rate of 1 gram 1% aqueous gel/2 ml isopropanol, whereafter the slurry is filtered through a synthetic straining cloth. After the filtration has gone about as far as it can, the wet filter cake is again dispersed in fresh amount of isopropanol at the same ratio, stirred, filtered and dried in an oven. Thus, the cake obtained after the first filtering step is still described as wet and must be treated once again with isopropanol.
"According to European patent EP-0859011, a transparent viscous gel consisting of microfibrils of cationic cellulose is prepared, whereafter the drying can be performed by adding isopropanol or ethanol, or any other solvent having dewatering capability, to a 3% aqueous gel, whereafter the dehydrated microfibrils of cationic cellulose are recovered by filtration and dried in an oven. In this way a powder that maintains the rheological properties when redispersed in water is obtained. In Example 7 of this patent, a 3% viscous transparent gel of microfibrils of cationic cellulose is precipitated in isopropanol, filtered and dried in an oven to obtain dry product that recovers its original rheological properties upon addition of water.
"In our experiments this protocol has been tested and found problematic. NFC is able to form colloidally stable dispersions also in alcohol media and filterability has not been improved as much as could be expected.
"The article by Capadona J. R. et al. 'A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates', Nature Nanotech. 2, 765-769 (2007) describes gels made of nano-scale cellulose whiskers which are obtained through acid hydrolysis of tunicate mantles. The whiskers exist initially in aqueous dispersion and they are made to an organogel in a sol-gel process through solvent exchange with a water-miscible solvent, whereafter the gel is filled with a matrix polymer by immersing the gel in a solution of the polymer and dried. During the gel-forming step acetone was introduced on top of the aqueous whisker dispersion without mixing the layers. The acetone was exchanged daily and the acetone layer was gently agitated to promote the solvent exchange. After some days the acetone organogel was obtained, placed into a solution containing a polymer and dried after removal from the solution. The article also reports the use of acetonitrile, ethanol, methanol, isopropanol and tetrahydrofuran as solvents for making the organogel. The gel forming step through solvent exchange takes typically many days."
In addition to the background information obtained for this patent application, NewsRx journalists also obtained the inventors' summary information for this patent application: "It is a purpose of the invention to provide a new method where the nanofibrillar cellulose can be dewatered by water-miscible solvents in a process which is faster and offers more possibilities for the further processing of the nanofibrillar cellulose to final products.
"It has now been found that water can be extracted from NFC hydrogels using a water miscible solvent, e.g. ethanol as an extraction agent by a practical method which reduces the drying time and makes it possible to manufacture a variety of products starting from the NFC hydrogel.
"The core idea of the invention is to carefully distribute NFC gels, as obtained from manufacturing process, into a water miscible solvent so that they remain as physical entities. This can be done by extruding or spraying the gel to the solvent to make e.g. 'worm' or 'sphere'-like objects out of the gel, or by supplying the gel in larger 'blocks' to the solvent and breaking them to smaller particles in the solvent. These blocks can be crumbled into smaller entities in the solvent volume mechanically for example by agitating. The form of the entities depends on the way of supplying the NFC into the solvent. With a proper technique, the water in the NFC hydrogels can be completely changed into e.g. ethanol. In the second stage of the process, the entities are separated physically from the solvent volume, and the remaining solvent and possible residual water is removed from them by a suitable drying method, for example in vacuum and/or elevated temperature (higher than 25.degree. C.), and dry NFC is obtained. The drying can take place also by pressure filtration. A combination of these methods is also possible, for example initial drying by pressure filtration and final drying by vacuum and/or elevated temperature. The total process time starting from the supply of the hydrogel into the solvent and ending in obtaining the dried product after the drying step is of the order of some hours, preferably not longer than two hours. During the drying the entities also shrink to their final dimensions.
"The hydrogel is introduced in the organic solvent carefully so that it remains coherent and does not become dispersed, that is, a phase boundary of the gel against the solvent volume exists the whole time after the contact of the hydrogel with the solvent, although the phase boundary may change due to disintegration of larger gel entities, such as or blocks, into smaller ones in the solvent.
"When the water is exchanged with the solvent in the solvent volume, the physical entities keep their original dimensions and the shapes where the gel was initially supplied to the solvent volume, or the dimensions and shapes change to the final dimensions and shapes of the entities due to the disintegration of the original gel entities. In both cases a small shrinkage may also occur. During this solvent exchange process, the NFC nanofibrils contained in the gel are stabilized to a coherent structure, where, however, a certain porosity exists. Due to this porosity the evaporation of the solvent is easy after the physical entities are separated from the solvent volume.
"The NFC product obtained after the separation of the entities from the solvent volume can also be left with the remaining solvent and possible residual water in its structure, and it can be subsequently packed and shipped later to the end user as a solid 'semi-dry' product.
"The organic solvent that is used for the solvent exchange with the water of the hydrogel is any solvent that is miscible with water and preferably has moderate polarity. Suitable solvents are water-miscible alcohols including but not limited to methanol, ethanol and isopropanol, as well as dioxane and THF. The exchange of the water with the solvent leads to an enforcement of the hydrogen bonds between the NFC fibrils and to mechanical stabilization of the physical entities formed.
"Preferred solvent is ethanol, which has low toxicity, low heat of evaporation (904 kJ/kg vs. 2256 kJ/kg for water) and exothermic mixing reaction with water (-777 J/mol at 25.degree. C.) which lowers the energy demand.
"The main advantages of the invention are the following: NFC can be dried without lengthy filtering operations and without need to evaporate water, because organic solvent having a lower heat of evaporation is evaporated instead; dried NFC can be transported in a form of particles which are easy to handle and can redispersed in water at the site of use.
"During the introduction of the hydrogel into the solvent, the stirring or agitation, if used, must be careful in order to avoid the dispersing of the NFC hydrogel into the solvent so that the hydrogel is maintained as discrete physical entities.
"One possibility is to supply the hydrogel through a port that comprises several orifices or nozzles, and the shape of the entities in the solvent volume is thus determined by the shape of the orifices or nozzles and by the supply rate of the hydrogel into the solvent. The port is preferably immersed in the solvent volume so that the hydrogel is immediately in contact with the solvent after issuing from the orifices or nozzles. The supply rate can be even and continuous, in which case continuous physical entities are formed in the solvent, or intermittent, which creates discontinuous entities, for example beads.
"Another possibility is to disintegrate the hydrogel while in the solvent. In this case the hydrogel is supplied in larger blocks into the solvent volume, whereafter it is disintegrated to the physical entities mechanically so that the phase boundaries remain between the solvent and the NFC. This can be done for example by the blades of the agitator. The NFC hydrogel can for example be disintergrated in a smaller volume of the solvent, whereafter this solvent volume together with the physical entities formed is combined with a larger, final volume of the solvent and the solvent exchange process is completed in this volume of solvent.
"According to one embodiment of the invention, the physical entities obtained are comminuted to a smaller size, for example particulate matter or powder which is an easily transportable form of dry NFC, which has retained its rheological properties. This comminuted form of the NFC can be used as a redispersable dry product, which the end user can make to a gel of desired concentration by simply adding water, preferably at the site of the use of the gel.
"According to another embodiment, the physical entities are separated from the solvent volume, the remaining solvent and possible residual water is removed to form a dry product, or left in the entities to form a 'semi-dry' product. This dry or 'semi-dry' product where the physical entities are in their original shape and possibly only shrunk due to the drying, is another transportable product which is redispersable to a gel at the site of use by adding water.
BRIEF DESCRIPTION OF THE DRAWINGS
"The invention will be explained in the following with reference to the enclosed drawings, where
"FIG. 1 is a scheme of a process where the method according to the invention is employed;
"FIG. 2 is a graph showing water content of ethanol in course of solvent exchange;
"FIG. 3 is a graph showing the redispersion behavior of dried NFC obtained by a method according to the invention;
"FIG. 4 is a graph showing the redispersion behavior of dried NFC of another type;
"FIG. 5 is a graph showing the redispersion behavior of dried NFC of a third type;
"FIG. 6 is a microscope picture of dry unground NFC made according to the method of the invention;
"FIG. 7 is a stereomicrograph of dry ground NFC made according to the method of the invention;
"FIG. 8 shows samples of NFC in ethanol right after the solvent exchange, and
"FIG. 9 shows a graph of solvent removal and dry solids increase of NFC in pressure filtration."
URL and more information on this patent application, see: Laukkanen, Antti; Teirfolk, Jan-Erik; Nuopponen,
Keywords for this news article include: Acetone, Alcohols, Hydrogel, Microfibrils, Ethanolamines, Ketone Bodies, Organic Chemicals, Extracellular Matrix, Polyethylene Glycols,
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